The present invention relates to a nuclear medical diagnostic apparatus for externally detecting gamma rays emitted from RI (Radio-Isotope) injected to a subject and generating an RI distribution in the subject on the basis of the detection data.
Nuclear medical diagnostic apparatuses are classified into planar image-type apparatuses which obtain an RI distribution on a projection plane and ECT-type (Emission Computed Tomography-type) apparatuses which obtain an RI distribution on a slice. The ECT type nuclear medical diagnostic apparatuses include a SPECT (Single Photon Emission Computed Tomography) apparatus using single photon RI such as 99mTc or 111In, and a PET (Positron Emission computed Tomography) apparatus using positron RI such as 11C or 13N. Recently, apparatuses serving as both SPECT apparatuses and PET apparatuses have appeared. These apparatuses in general will be called nuclear medical diagnostic apparatuses hereinafter.
A conventional nuclear medical diagnostic apparatus has an Anger type radiation detector. As shown in FIG. 1, the Anger type radiation detector is comprised of a collimator 10, scintillator 11, lightguide 12, and plurality of PMTs (PhotoMultiplier Tubes) 13. When gamma rays come incident on the scintillator 11, fluorescence is generated at the incidence position. The fluorescence is detected by the plurality of PMTs 13. The sum of output signals from the plurality of PMTs 13 reflects the gamma ray energy. Among events wherein radiation is detected, an event derived from radio-isotope injected to the subject is selected on the basis of the total energy. The selected event is counted in association with the incidence position of the gamma rays. The incidence position of the gamma rays is calculated as, e.g., the barycentric position of energy.
Gamma rays having a high energy of 511 keV, which is derived from positron, often cause the Compton scattering in the scintillator 11. Because of the Compton scattering, energies E1 and E2 (incidence energy E0=E1+E2) are absorbed at two positions P1 and P2 almost simultaneously. One of the two positions P1 and P2 is the true incidence position.
Conventionally, however, the incidence position of the gamma rays is calculated as the barycentric position of an energy which does not coincide either of the two positions P1 and P2 or naturally the true incidence position. In other words, all the events wherein scattering occurs in the scintillator are counted as having occurred at erroneous positions. In addition, conventionally, whether scattering occurs in the scintillator cannot be determined.
In a PET-exclusive apparatus having a BGO (bismuth germanium oxide) detector for performing block detection as well, when gamma rays are scattered among blocks of the BGO detector, the PET-exclusive apparatus cannot separate events that occur simultaneously to obtain the accurate positions of the events by calculation. Accordingly, a decrease in counting precision cannot be avoided.